1. Field of the Invention
The present invention relates to a nanowire transistor and a method for fabricating such a transistor. More particularly, the present invention relates to a transistor including a nanowire with a core-shell structure, in which a semiconductor core portion is coated with a gate insulating film. Such a nanowire transistor can be used in an electronic device or a micro device to be fabricated on a flexible substrate using thin-film transistors.
2. Description of the Related Art
A conventional active-matrix-addressed LCD or organic electroluminescent (EL) display is formed by arranging a number of pixels in matrix on a glass substrate. Specifically, on the glass substrate, arranged are scan lines and data lines that run in columns and rows. And each pixel can be electrically connected to by way of a transistor (switching element) that is arranged near the pixel. Since a single crystalline semiconductor layer cannot be formed on a glass substrate according to any of the current techniques, a thin-film transistor (TFT), which is made of an amorphous silicon or polysilicon film that is not single crystalline, is formed on the glass substrate and used as a switching element for controlling the pixel.
Some people have proposed using a plastic substrate that is more lightweight and more flexible than a glass substrate. However, such a flexible substrate softens at a lower temperature than a glass substrate. For that reason, even the amorphous silicon or polysilicon film cannot be formed on such a flexible substrate. That is why organic semiconductor TFTs and nanowire transistors have been developed as transistor devices that can be fabricated by lower-temperature processes.
In “High Performance Thin Film Transistors Using Semiconductor Nanowires and Nanoribbons”, Nature, Vol. 425, 2003, p. 274, disclosed is a nanowire transistor that uses a nanowire of a single crystalline semiconductor, not a semiconductor thin film, as the channel region of the transistor. Such a nanowire transistor may be fabricated in the following manner.
First, a substrate to produce crystal growth thereon is provided separately from a substrate for supporting transistors, silicon nanowires are grown on the former substrate, and then the silicon nanowires are removed from the crystal-growing substrate. The silicon nanowires are made of single crystalline silicon and have excellent crystallinity. To grow nanowires, a high-temperature process needs to be carried out at a temperature that is too high to withstand for a normal flexible substrate. As the crystal-growing substrate, however, a substrate that can withstand such a high-temperature process is used.
Next, a great number of silicon nanowires thus obtained are dispersed in a solution. Subsequently, the silicon nanowires, dispersed in the solution, are arranged on a flexible substrate and then electrode structures are formed thereon, thereby completing nanowire transistors on the flexible substrate. The electrode structures may also be formed at a relatively low temperature that a plastic substrate can withstand.
In the transistor fabricated by such a method, its channel region is made of single crystalline silicon. For that reason, even on a flexible substrate, the transistor realizes very high performance including an electron or hole field effect mobility or more than 100 cm2V−1s−1.
Hereinafter, the configuration and problem of the conventional nanowire transistor will be described with reference to FIGS. 1 and 2. FIG. 1 is a perspective view illustrating a conventional nanowire transistor. FIG. 2(a) is a top view of the nanowire transistor shown in FIG. 1 and FIG. 2(b) shows a cross-sectional structure thereof as viewed on the plane A-A′.
The nanowire transistor shown in FIG. 1 includes a flexible substrate 1, a gate electrode 2 on the flexible substrate 1, a gate insulating film 4 that covers the gate electrode 2, nanowires 3 arranged on the gate insulating film, and source/drain electrodes 5 and 6.
The nanowire transistor shown in FIGS. 1 and 2 has the following problems.
Specifically, on the flexible substrate 1, the process temperature is restricted, and therefore, the gate insulating film needs to be formed at a temperature of 200° C. or less, for example. That is why the gate insulating film should be either an organic insulating film formed by a coating process or an inorganic insulating film formed by a low-temperature CVD process or a sputtering process. However, the organic insulating film and the inorganic insulating film, deposited at a low temperature, have too low breakdown voltages to achieve high reliability as a gate insulating film.
On top of that, the gate insulating film 4 formed by such a low-temperature process has increased interface levels and decreased carrier mobility in the interface between the film 4 and the channel, which is also a problem. Besides, as there are fixed charges and mobile ions in the organic insulating film, the threshold value may shift, the current-voltage characteristic may produce hysteresis, and the variation in characteristic may increase.
In order to overcome the problems described above, the present invention has an object of providing a novel transistor and a method for fabricating such a transistor. Another object of the present invention is to provide an electronic device using such a nanowire transistor.